The present technology relates to a heterojunction field effect transistor (HFET) with enhancement mode (normally off) and depletion mode (normally on) regions which are serially connected.
HFETs (including high electron mobility transistors (HEMTs) or metal-insulator-semiconductor-HFETs (MISHFETs) or double-channel HFETs/HEMTs/MISHFETs or dual-Channel HFETs/HEMTs/MISHFETs, or thin-body (SOI, finFET, tri-gate, gate-all-around, etc.) HFETs/HEMTs/MISHFETs, may be used, for example, as switching devices. Such devices are typically formed of III-V semiconductors and achieve very high mobility by having an undoped channel region. In conventional HFETs, the device is described as “normally on”; i.e. the threshold voltage, also sometimes referred to herein as pinch-off voltage, is zero or negative, and the channel conducts electric current with little or no bias applied between source and gate. For power electronics applications, a normally off device is strongly preferred, for safety, energy conversion and circuit design reasons. For example, a normally on device will allow a significant amount of power to flow between source and drain in the event of a failure leading to a floating or grounded gate terminal.
It is known, in power applications, to arrange a normally off device serially connected to a normally on device. A conventional normally off device may be formed of silicon, while a normally on device is typically formed of other materials, for example III-V semiconductors. It can be complex and expensive, however, to combine different materials in a single fabrication process. It would be advantageous to form a normally off device serially connected to a normally on device using a streamlined fabrication process.